Aluminum chromating process



United S t-ates ALUBIIN UM CHROMATIN G PROCESS William S. Russell, Royal Oak, and James L. Van Vliet, Huntington Woods, Mich.

Application February 9, 1959' Serial No. 791,800

16 Claims. (Cl. 148-616) I No Drawing.

This invention relates to the art of coating aluminum ings has been the subject of extensive research in the past. It was early discovered that the hexavalent chromium ion was the key to corrosion protective coatings and that simple aqueous hexavalent chromium solutions required modification to include activator ions to enable attack on an aluminum surface and the consequent formation of adherent coatings in a reasonable length of time. U.S. Patent 2,114,151 taught that aqueous hexavalent chromium solutions could be activated satisfactorily with one or more of the halo-gen and the ferricyanide ions. It was discovered later that aqueous hexavalent chromium containing solutions could be advantageously combined with phosphates to form a combination chromium-phosphate coating having certain improved characteristics, but that such solutions required the presence of the fluoride ion and careful control and maintenance of the proportions of the hexavalent chromium ion, the fluoride ion, the P ion as well as the acidity of the solutions. U.S. Patent 2,438,877 teaches that adherent green coatings are obtained on aluminum by using an aqueous solution consisting of 0.9-12.5 grams per liter flluorine, 3.75-60 grams/liter CrO 2-285 grams/liter P0 when the ratio of FzCrO is in the range of 0.135-0.405' and the pH of the solution is in the range of 1.6'to 2.2 as measured by the lowest value indicated by a glass electrode pH meter within the first ten minutes of immersion of the electrode in the solution. Somewhat later U.S. Patent 2,494,910 taught that satisfactory coatings are formed on aluminum or aluminum alloy surfaces with much less acid solutions than those employed in U.S. Patent 2,438,877, namely in the range of pH of 3.2 to 7.0. These aqueous solutions consisted of 1.5 to 300 grams/liter P0 ion, 3.5 to 150 grams/liter ClgOq ion, 0.75 to 95 grams/liter F ion and operated satisfactorily at room temperature so long as the ratio of Cr O to F remained in the range of 1:0.375 and 1:5.45 and pH remained in the range of 3.2 to 7.0 as measured colorimetrically. Both of these patents teach that the P0 ion, the dichromate ion and the fluoride ion may be introduced as the acid or as the alkali metal salt and where the alkali metal salt is used any needed adjustment of acidity to get into the specified range is obtained by the use of a conventional acid or alkali. These patents also teach that a variety of cations may be present and are harmless except as their presence may cause loss of P0 F or CI'zOq ions by precipitation of salts. The cations so listed as harmless in reasonable quantity include aluminum, trivalent chromium, zinc, copper, mananese, iron, nickel, cobalt, calcium, barium, strontium, tin and others, while excessive quantities of aluminum or calcium are indicated to cause loss of fluoride by precipitation but ar n't" 0 tities of coating chemicals than have been required with 2 to be otherwise harmless. It has been found that the continuous operation of said solutions in the pH range of 1.6 to 2.2 produces excessive quantities of sludge.

The present invention is based on the discovery that aqueous hexavalent chromium solutions containing phosphate and fluoride ions operate more efficiently and subestantially are non-sludge forming at higher acidities than a pH of 1.6, for example at a pH between about 0.8 and 1.5. Additionally, it has been found that as aluminum is coated in an aqueous solution having a pH in the range of 0.3-1.5 and containing quantities of phosphate, fluoride and dichromate ions satisfactory to produce a coating on aluminum when the solution is a new solution, continued use causes aluminum and trivalent chromium ions to build up in the solution. Moreover, at the fluoride and CrO concentrations specified in- U.S. 2,438,877, the presence of these ions in the solution in quantities less than is required to cause sludging, or their precipitation as salts, is detrimental to the coating form-ing ability of the solution and the resultant coating weight gradually decreases. In order to maintain coating weight and the desired appearance of the coating in these more acid solutions it is necessary to adjust the coating forming ingredients in the solution, particularly the proportions of fluoride, and dichromate ions until the ratio of fluoride to dichromate ion, calculated as CrO is well above 0.405.

It is the principal object of this invention to provide a substantially non-sludging process for continuously coating aluminum or aluminum, alloy surfaces with an adherent coating having good corrosion resistance and utility as an aid in cold forming operations and as a base for paint and vitreous enamel.

Another object of this invention is to provide improved operating, make-up and replenishing solutions suitable for use in practicing the method of this invention. A

A further object of this invention isto provide a solution which is capable of tolerating greater quantities of aluminum and trivalent chromium ions than heretofore known solutions, and a method forcontrolling and replenishing such solutions so that continuous operation is made feasible without sludging and without the necessity of selectively removing aluminum and/or trivalent chromium ionsfrom the solution.

Another important object is to provide a process for forming corrosion resisting and paint base coatings on aluminum that is suitable for continuous spray application without encountering deposition of sludge on heating pipes, etc.

Other objects include the provision of a process which concurrently improves the rate of formation of adherent coatings on aluminum and aluminum alloy surfaces and extends the range of variation of concentration of the coating forming ingredients which can occur without derirnentally affecting the adherence or corrosion resistance quality of the resulting coatings; the provision of a process which effects improved consistency of weight, adhesion and color in the coatings which are produced; and the provision of a process which continuously produces a given weight of integral, adherent coating on aluminum with significantly reduced quanheretofore known processes. Broadly stated, the method of this invention comprises the steps of contacting the surface of aluminum or its alloys with an aqueous acidic solution containing phosphate ions, fluoride ions and dichromate ions in appropriate proportions, at a temperature and for a time suflicient to cause an insoluble, adherent coating to form on the surface, the solution at all times having a pH in the range of 0.8 to about 1.5. As the solution is used and impurities, including aluminum and trivalent chromium,

Patented Mar. 15, 1966- the usual nozzle plugging and increase therein the process may include the further step or steps of adding ingredients so as to adjust the relative proportions of fluoride and dichromate ions to pre-selected values and maintain the acidity in the stated range. In-

creasing quantities of aluminum in such solutions has a softer, and somewhat less adherentthan other coatings but it still has utility as a base for painti In the formation of the coating the"aluminumidissolves from the surface being'coat'ed in an amount gre'ia't'erthah" that which remains'in the coating and the excess remainsf in the coating solution; Similarly. during coating some of the hexavalentchromium ions"- the' solution are re'- duced'to trivalent chromium ionswhi'ch also remain, and while these impurity ions cannotbe avoided inthe solotion, the presence of even minor quantities of other cations such as sodium, potassium, ammonium, zinc,copp' er, man ganese, iron, nickel, cobalt, calcium, barium, strontium, tin, etc. should be avoided. Such other cations function in conjunction with increasing quantities" of trivalent chromium in the solution to tend to raise the pH of the solution and'a's the quantity'of such'ionjs, panicul'any the alkali metal ions, sodium, potassium and includihg' am monium, build up in the solutionftli'e effect is additiveand the difiiculty' of maintaining the desiredlac'idit'y is increased. Even though small quantities of such ions can tolerated in'the new solution having an acidity within the range'of'illi to T.5','the pr1esenceof such small quas ties is undesirable" because" they exert their detrimental effect after the bath becomes aged and the trivalent chromium has" increased in the solution to an extent that its presence has an appreciable effect on the maintenance of the desired acidity. The presence of such ions in quantities suflicient to cause the pH to migrate toward i an etch, and from the comb ined standpoints of ease of maintenance of the solution and efiicient operating condition, economy, and from the standpoint of obtaining the widest coating weight range from the bath, it is preferred to employ relative proportions of the P CrO aluminum and P which approach the following preferred limits. The P0 ion is preferably kept within the range of 20 to 80 grams/liter, the CrO ion is kept between about 5 and 30 grams/liter, the aluminum ion is kept between about 1 and 30 grams/liter and the fluoride ion is kept between about 2.5 and 69 grams/liter. In all cases the pH of the solution should be maintained within the range of 0.8 and 1.5 and preferably within the range and above'the upper limit of 1.5 manifests itself in the formation of undesirable insoluble fluoaluminate precipi fate or sludge. For this reason it is undesirable to make up or replenish the operating solutions with salts containingsuch foreign" cations and it is one of the advau tag'eous features of this invention that. the P0 ions, the CrO ions andthe fluofide ions are introduced only in the form of the respective acids to maintain theoperating solutions in eflic'ient coatingv condition in a continuous process, for example phosphoric acid, chromic acid, bydrofluoric acid andfl'uoa'lur'ninic acid, H AlF p The P0,, ion content of the solutions of this invention should be in the range of aboutSgrams/litertdabout 150 grams/ liter and is preferably introduced from make-up or replenishing materials asphosphoric acid. The dichromate ion, calculated as chromic acid, CrO can satisfactorily vary between about 2.5 and about 62 grams/liter and preferably is introduced in the solutionsin the form of chromic anhydride or chromic acid. The sum of the concentration of C'rO and P0 ions is prefer ably above a minimum of about if) grams/liter. When the sumof these ions falls below about 10 grams/liter the pilot the solution tends to numerically exceed 1.5 and the tendency for sludgin'g and: the formation of loose coatings is increased. The'fluoride' ion concentration ofthe bathmay satisfactorily vary from a minimum of about 2,5 to about[123 grams/liter and is preferably introduced into the solutions in the form of hydrofluoric acid. As the extreme limits of concentrationof P0 CrO and F are approached, and particularly where the opposite extreme concentrations are selected for use in a single installation, the resulting coatings' tend to become less desirable in one or more of their characteristics, that is, they become softer, looser or the coating action approaches that of Although foreign cations, as above indicated, are undesirable the solutions of this invention will continue to form satisfactory coatings in the presence of small quantities of them and in the presence of anions such as sulfates, nitrates and chlorides, and for this reasons it is possible topartially regulate the desired acidity by introducing a strong mineral acid such as hydrochloric, sulfuric or nitric acid, if desired. It is preferred, however, that the acidity of the solution be derived from phosphoric, c'hromic and hydrofluoric acids, and in this case the major acidity effect is derived from the phosphoric and chromic acids.

The presence of fluoride ions in the solutions comp icat'es the measurement of the hydrogen ion concentration, or pH of the solutions, but it has been found that a reasonably accurate and reproducible measurement of the hydrogen ion concentration can be obtained and it is to be understood thatthe pH values referred to in this specification andlin the appended claims refer to hydrogen: ion concentrations determined by the use of the following procedure; The pH measurements are taken using an electrical pH meter employing a glass electrode and a calomel'electrode by immersing' the electrodes'in aportion of the solution and observing theindicated value for a ten minute period and taking the lowest value indica'tediat any time during that interval. Although there is some fluctuation in the indicated pH value during the ten minute observation period, the range of fluctuation in the more acid solutions of this invention is noticeably les's than is characteristic of less acid solutions of otherwise'similar type and generally reaches the lowest value ii li a" few seconds to one minute. In order to obtain reproflucible results, however, because the glass electrode is subject to degenerative attack by the fluoride in the solution, the electrode upon removal from the solution should be immediately rinsed'in a solution of five normal hydrochloric acid by inserting it in that solution for two m nree minutes and thereafter rinsing the el'ectrode in pure water. The electrode is then kept immersed in ure Water until the next reading, and preferably is checked against a standard buffer before use. Under this procedure a glass electrode has been found to give reproducible results having a range of :0.05 pH, whereas failure to counteract the fluoride attack will cause the electrode to give inaccurate and non-reproducible readings' after only a' few uses, sometimes even after only one reading. I w

The solutions of and the method of this invention are useful in the formation of coatings on pure aluminum and the' alloys thereof including extrusions, cast, wrought and sintered alloys... The process has been found to satisfactorily coat all of the commercially available wrought aluminum alloys which have been tested and specifically ihcludingllllil, 2014', 2017, 2024, 3093, 3004, 3005, 5052,. 5056-, 6061, 7072 and 7075 While the compositions ofthesealloys vary and each is adapted for a particular application, they are all alloys in which aluminum is the principal ingredient.

Whilethe surfaces to be coated should be free of grease, dirt, oxide films and the like, special cleaning is not required: prior to the'u'se of the process of this invention. Conventional cleaning with mild alkali cleaners and/or acid or caustic treatments, as needed, by employing conventional techniques are satisfactory to prepare the surfaces to be coated.

The solutions can be satisfactorily applied at temperatures varying from room temperatures to as high as about 180 F. and in general the rate of coating formation increases as the temperature increases. However, as the solutions become more acid and the fluoride concentration increases metal attack is increasedto the point at which the resultant coating approaches a loose powdery condition, so that in general the use of temperatures between about l20 F. and 140 F. are best for the majority of applications. At these temperatures the rate of coating formation is sufficiently fast to produce any desired coating weight in a reasonably short time, for example, five seconds to three minutes, and the use of higher temperatures is less desirable in comparison because of the increase in cost and the relatively minor increase in coating rate formation which accompanies the same. When using solutions which are relatively high in fluoride concentration, the lower temperatures are preferred, and it is to be understood that the above given limits of F, CrO and P; are sufficiently broad to enable the use of temperatures as low as room temperature in a continuous process, if deired.

The solutions may be satisfactorily applied by spraying the surface to be coated with the solution, by dipping the part in the coating solution or by brushing or flowing the solution over the surface. Because of the substantial non-sludge forming characteristics of the solutions of this invention, application of the solutions by spraying is particularly desirable.

- As above indicated, the use of the solution of this invention to form coatings on aluminum surfaces dissolves more aluminum, during the coating formation, than is contained in the coating and aluminum builds up in the solution. It has been found that in order to maintain the solution in continuous operative coating formation condition, it is important to regulate the content of fluoride in the solution relative to the aluminum which is dissolved in that solution. Unless the fluoride concentration is raised as the aluminum content increases in the bath, the coating ability of the solution decreasesto the point that the desired coating weight is not obtained and for continuous operation to produce coatings of substantially constant weight and color, the relationship of fluoride to aluminum should bemaintained in the proportions set forth below in Table I. Percentages throughout are weight/ volume.

TABLE I Fluoride, Percent in Solution Aliuninum, Percent in Solution Minimum Maximum Table I sets forth the permissible variation in the fluon'de content representing the extreme conditions of variation which will produce satisfatcory coatings in a continuous steady state operation as the other bath constituents andacidity conditions are allowed to vary within the above given ranges, that is, the CrO P0 and pH. It will be appreciated, however, that for any given installation having a preselected pH and relaitvely constant concentration of CrO and P0 that there is an optimum value of fluoride concentration which produces the best coating. For example, it has been found that in a soluaeeag'tea tion operating at a pH between about 1.1 and 1.2 and having a CrO content of 14 g./l'. and a P0 concentration of 50 g./1. a fluoride concentration which is about 0.4% above the minimum values given above in Table I represents a good continuous working concentration for the fluoride to produce high quality, adherent, constant color coatings at a rapid rate over the entire range of aluminum concentration in the bath. While the-optimum fluoride concentration for other installations will vary between the minimum and maximum values an amount other than the 0.4% above the minimum for this specific bath, it will be apparent that the optimum concentration will in most cases be well above the minimum and somewhat below the maxirnum and that the optimum concenration can be easily establishedby one or two well selected tests under operating conditions, preferably be-' ginning approximately midway between the given mini-. mum and maximum in Table I for the analyzed concentration of aluminum in the steady state operating solution.

The concentration of fluoride set forth above in Table I and in other portions of this specification and in the appended claims refers to fluoride concentration determined by the Willard and Winter method described in Industrial and Engineering Chemistry, Analytical Edition, vol. 5, pp. 7-10, 1933, modified to employ the titration described by Rowley and Churchill in the same journal, vol. 9, pp. 551, 2, 1937.

- In accordance with this invention it has been found that the rate of increase in aluminum concentration in' the operating solution is dependent upon the weight of coating which is being formed on the surface treated and that in typical commercial operations aluminum in an amount of approximately 26% to 30% of the weight of the coating which is formed is continuously added to the solution, that is to say, if a Bil-milligram coating is being. formed on the aluminum surface the processing of each square foot of aluminum through the coating solution adds 26% to 30% of 30 milligrams of aluminum to the coating solution, or 7.8-9.0 milligrams of aluminum per square foot of surface processed through the solution. In any given installation continuous operation of the process depletes the coating solution by dragging out a portion of it on the work processed therethrough, and. it has been found to be feasible to maintain an operating solution in steady state operating condition by controlling the proportion of operating solution which is continuously removed as work is processed so as to balance the proportion of aluminum which is dissolving in the solution against that which is lost by the combination of drag-out and a preselected quantity of solution voluntarily discarded. As coating is formed dichromate ions, P0 ions and fluoride ions are consumed and in order to maintain the above given relative proportions of these coating forming constituents in the bath, it is necessary periodically, or continuously, to replace the coating solution with these materials by adding the corresponding acids to the solution. It will be apparent that if a portion of the operating solution containing a quantity of aluminum is continuously discarded and the only materials added to the bath are the acids ofthe respective coating forming constituents that the total concentra tion of aluminum in the solution can be thereby effectively controlled. It has been found that by discarding a relatively small quantity of the solution continuously and at times in addition to that which is normally dragged out on the work being treated that the solution can be maintained in balance and a steady state operating condition effectively maintained. As a guide to the control of a steady state operating condition by this procedure the following typical examples will be given. An operating solution contains 1.4% CrO 5% P0 0.7% fluoride and 0.33% aluminum, and it is desired to maintain these conditions in the bath whilea 30-milligram p r square foot coating is. formed on the aluminum-'ilge ing processed through the solution. The formation of 30-milligram coating persquare foot on the work being processed adds about 8.4 milligrams of aluminum to the bath per square foot and since the solution contains 0.33% aluminum. the removal of 2.5 ml. of thesolua tion per square foot of work treated will remove 8.4 milligrams of aluminum from the bath and the aluminum concentration will not increase. The removal of this quantityof treating solution is eifected by employing conventional methods of controlling drag out, and by successive aluminum analyses confirming the quantity being discardedrcmoves an amount of aluminumequal to that being dissolved'in the bath from the work.

- If this same solution'is operated at a temperature and for a suflicient time to produce an average coating Weight of 100 milligrams per square foot the formation of the coating will continuously add" 28 milligrams of; aluminum to the bath per square foot ofsurface treated; At the same aluminum concentration: in the bath the aluminum will not build up when the solution is discarded at the rate of 8.4 ml. of theprocessing solution per square foot of surface processed therethrough.

In the operation. of a continuous large scale process the initial stages of operation representing the aging period for the bath are the most critical to control and keep in a condition which will produce constant color and constant coating weight coatingsbecause the aluminum and trivalent chromium concentrations in the solu- The starting solution can be formulated to include trivalent chromium ions, as well, if desired. It is therefore possible to preliminarily formulate a solution which represents the solution in useduring steady state operating conditions and which thus eliminates all uncertainties regarding the aging period. Typical concentrated makeup materials suitable for this purpose may have the following composition:

Make-up formulation I Parts by weight 70 H PO (75%) 345 AlgOs-XHzO H O 480 When 80 pounds of this material is added to 100 gallons of water a solution is formed containing 0.7% CrO 0.1% A1, and 2.5% P and an operating solution is produced when hydrofluoric acid is added thereto to produce an F/CrO ratio of 0.5-1.4, namely 4.8 pounds to 14 pounds of 60% HF.

Make-up formulation II Parts by weight cro 280 H rd, 75% 1370 A1 O .xI-I O 30.5% Al) 8? When 292 pounds of this material is added to 100 gallons of water a solution is formed containing 2.8% CrO 0.3% A1 and 10% P0 HF added to this solution to produce 1.4% F makes a satisfactory solution.

Make-up formulation III Parts by weight 70 c oa I r rnro 690 Al O xH O(30 5%Al) 33 When 131- pounds of this formulation is added to 100 gallons of water, a solution is formed containing 0.7% CrO 0.1% Al and'5% P0 HF can be added to this solution in the range of quantities indicated as usableirt connection with Makeup formulation I. v

v Coatings of this invention, particularly the lighter: weight coatings, having primary utility as a base for paint can be rinsed in a dilute solution of chromic acid, if de sired, for example, a chromic acid solution having a concentration of about 0.02% to 0.10% CrO Such a rinsing with a chromic acid solution has a tendency to yellow the coating and except in applications in which the water available forfinal rinses is impure, the use of such a rinse is usually unnecessary.

The coatings are normally rinsed in pure water and thereafter may be dried in air or force-air dried at elevated temperature. Satisfactory drying of the coatings by elevated temperature drying can be effected at temperatures of between about 150 F. and 200 F. Temperatures above about 200 F. should be used only for veryshort times since prolonged contact at such temperatures causes the coatings to be loose. The coatings of this invention, particularly coatings having weights of a range of 200-800 m g/sq. ft. are useful as aids in deformation operations and can be used in conjunction with conventional lubricants, such as aqueous soap solutions, soapborax admixtures, etc. The lighter-weight coatings have also been found to be useful as a base for vitreous enamels, particularly coatings having weights in the range of about 10-70 rug/sq. ft. and for this use the coatings require merely to be dried and the vitreous enamels applied by conventional techniques and fired thereover.

The exampleswhich follow are intended. to illustrate typically operative formulations and conditions for the practice of the methods ofthis invention, but it is to be understood that the particular quantities of ingredients, conditions of operation and characteristics of the products contained therein are illustrative only and do notdefine the limits thereof, which limits have been broadly set forth hereinabove.

EXAMPLE I a short immersion in a dilute alkaline or alkali bath and water rinsed, were immersed in the solution, one ata" time, at the temperature of 135 F. for one minute and withdrawn. Upon withdrawal from the solution, the panels were squeegeed to remove excess solution and after rinsing and drying the coating weight of the twen-, tieth panel was found to be 250 mg./ sq. ft. and to be decreasing. To prevent decrease in the coating weight the fluoride concentration was then increased to a concentration of 0.26%, to. thus produce an F to CrO ratio of 0.185. Twenty additional similar panels were processed in the solution under the. same conditions and ad herent green coatings were produced on their surfaces, but the coating weight of the 40th panel was found to be only 200 mg/sq. ft. The bath was again adjusted by adding additional hydrofluoric acid to raise the fluoride concentration to 0.29% and additions were made to re-' plenish the chromic and phosphoric acids in an attempt to maintain the original concentration and thus the bath, after adjustment, had an F to CrO ratio of 0.21. 40 additional panels were processed through this solution and inspection of the th panel showed that the coating Weight had been increased back to theoriginal 250 mg./ sq. ft., the coatings having a comparable medium green appearance and being smooth and adherent.

seseaee By employing a similar procedure of continuing to coat panels in the solution and adjusting the fluoride con centration to maintain the cc :rting weight at approximately the 250 mg./sq. ft. weight, 440 panels were processed equalling 147 sq. ft. through the bath. At this point, an analysis of the solution indicated the aluminum to be 0.18%, F to be 0.65% and Cr+++ to be 0.08%.

Using the same conditions and procedures, additional panels were processed through the coating solution and periodic additions were made to replenish the CrO P and fluoride to maintain the coating weight on the panels as close to 250 rug/sq. ft. as possible. No other changes were made in the Operational procedure and the resulting panels in all cases had a generally similar appearance and had on their surfaces an adherent medium green coating. The change in concentration of the various ingredients in the operating solution as these panels Were'processed through the bath is set forth below in Table II.

TABLE II Cumulative No. Sq. Ft. Cr+++, Al, F, 13/010 of Panels Percent Percent Percent After 227 sq. ft. of surface had been processed through the bath, the coating solution was analyzed and found to contain 0.82% fluoride, 1.39% CrO and to have a pH of'1.08.

' EXAMPLE II i This example is intended to illustrate the rate of aluminum and trivalent chromium buildup in an operating solution of the type illustrated in Example I except that it is operated at a pH in the range of 1.7-1.9, when started free of aluminum and controlled to produce a coating weight of 250 mg./ sq. ft. I

One gallon of an aqueous, acidic solution was prepared to contain 1.4% CrO P0 and 0.38% F and upon determination was found to have a pH of 1.1. This solution was heated to 135 F. and a plurality of 4" x 6", #3003 preliminarily cleaned aluminum panels were immersed in the solution, maintained therein for one minute, withdrawn, water rinsed and air dried. The coatings which were adherent and medium green in color were stripped from the surface and upon weighing were found to have a weight of 417 ing/sq. ft.

Suflicient sodium hydroxide was added to raise the pH of the solution to 1.9. Similar aluminum panels immersed in this modified solution were coated in one minute, with a coating having a weight of 544 mg./ sq. ft. but the coating was observed to be dusty and non-ad herent. Additional similar panels were processed through the solution and the coatings formed on the surface of each was observed. It was not until after 44 additional panels had been processed through this solution that the coatings became adherent. On the 45th panel a relatively adherent coating was obtained, generally medium green in color and it was found to have a weight of 341 mg./sq. ft. The solution was analyzed and found to contain 0.024% aluminum and 0.008% trivalent chromium. Twelve additional panels were processed through the bath and after this additional aging the coating weight was found to be 250 mg./ sq. ft. After 90 panels had been coated, for each sq. ft. of metal surface processed, the solution was replenished with 0.83 grams sodium bifluoride which assayed 54.7% F. Additional panels were processed through the solution under the same conditions, and after processing 81 panels through the bath, the bath was analyzed and found to contain..0.03% aluminum, 0.02% trivalent chromium, andto have a pH of 1.75. After 30 sq. ft. of surface was processed through the bath, the presence ofsludge in the solution was'definitely iii through the solution, the solution was analyzed and found to contain 0.026% aluminum, 0.036% trivalent chromi-i um, 1.4% CrOg, a fluoride concentration of 0.37% and" Processing was continuedan F/CrO ratio of 0.27. through the solution under the same conditions, and after 51 sq. ft. of'surface was processed through the solution,"

an analysis of the solution showed the aluminum content to be 0.02%, the trivalent chromium content to be 0.05,

Additional panels were processed through the bath un-" til sq. ft. had been processed therethrough' and-an.

analysis of the solution-indicated the aluminum .concen-' tration to be 0.005, the trivalent chromium concentration to be 0.084, CrO to be 1.4, F to be 0.34% .and an F/CrO ratio of 0.24. The pH was 1.76.

Additional panels were processed until a total of 140 sq. ft. had been processed through the bath andan analysis of the solution indicated that the aluminum concentration was 0.004%,the trivalent chromium con-' centration was 0.144%, CrO was 1.4%, F was 0.31% and the F/CrO ratio was 0.222. After 154' sq. ft. was processed through the solution an analysis of the solution showed the solution to contain 0.006%"aluminum, 0.16%..

trivalent chromium, 1.4% CrO a ratio of F to CrO of 0.22'andIto have a pH of 1.71. The coating formedon'the panels. processed between 40 sq. ft. and. 154.sq'..-.ft. were adherent, medium green in color and averagedabout; 250 mg./sq. ft. in weight. The sludgingcontinued and it is apparent that the aluminum and some fluoride were lost from the solution in the sludge. During this periodv of operation a quantity of approximately grams of dried sludge was produced, and an inspection of this: sludge indicated it to be an admixture of sodium fluoaluminate and chromium phosphate having an analysis of 55% F, 11% Al, 1% trivalent chromium and 2.8% P0 At the end of processing 154 sq. ft. of surface through the solution, 103 grams of sodium bifluoride had been added to the solution in order to maintain the coating action suflicient to produce anaverage coating weight of 250 mg./sq. ft. During this same processing, sufiicient chromic acid and phosphoric acid were added, to maintain the original concentration of 5%. P0 and 1.4% of CrO A second one gallon solution was prepared to contain 1.4% CrO 5% P0 0.24% fluoride and upon analysis was found to have a pH of 1.08. Similarly cleaned 4" x 6", #3003 aluminum panels were processed through this solution for one minute at F., the fluoride concentration being gradually increased to maintain a coat ing weight on the panels of about 250 mg./sq. ft. until a total of 124 sq. ft. of surface had been processed therethrough. At the end of this time, the bath, upon analysis, was found to contain 0.18% aluminum, 0.65% fluoride, no sludge, and during this period of operation a total of 26.0 grams of 60% hydrofluoric acid was added. In comparison with the operation of the otherwise similar solution in the pH range 1.7-1.9, it is apparent that comparable quantities of coating were formed on an equivalent surface area with'the consumption of approximately one-third the quantity of fluoride ion which was required with the numerically higher pH solution.

EXAMPLE III This example is intended to illustrate the effect on coating weight caused by varying the hydrogen ion concentration of a coating solution comparable to'an aged solution or one which had been in operation for an extended period of time.

A coating solution was prepared to contain- 0.16% aluminum, 0.7% fluoride, 1.4% CrO and 5% RQ anQ;

thus to have an F/CrO ratio of 0.5. The pH ofthis standard bath was found to be 1.02. Portions of this bath were separated and to each portion an increasing. quantity of sodium hydroxide was added to modify thep I-Iof the solution and panels were processedtherein and; the, appearance, the coating weight and formation of sludge; were noted.

. In the first portion ofthe solution, a precleaned panel of, comparable type to thatdescribed in Example I was immersed in the solution at a temperature of- 120 F. for 4; minutes, withdrawn, rinsed, dried, inspected and the coating stripped and weighed and it was found that the coating had a weight of 480 mgjsq. ft. was green in color and adherent. A second portion of the solution was modified by adding thereto 17.8 ml. of 50% sodium hydroxide per gallon of solution and the resulting solution had a pH of 1.23. Panels processed in this solution for 4 minutes at 120 F. were coated with a coating having aweight of 436 mg'./sq'. ft. A third portion of the standard solution was modified to contain 35.6 ml. of 50% sodium hydroxide per gallon of solution to thus produce a solution having a pH of 1.34. A panelprocessed in this solution was coated with a coating weight of 244 mg./sq. ft. after four minutes at 120 F. A fourth portion of the standard solution was modified to contain 5 3.4 m1. of 50% sodium hydroxide per gallon of solution to thus form a solution having a pH of 1.52. The addition of the sodium hydroxide to this solutionpfoduced sludge which was visible in the solution anda panel processed in: the solution for 4 minutes at 120 F. produced a coating weight of 100 rug/sq. ft. A fifth portion of the standard solution was modified to contain 71.2 in]. of 50% sodium hydroxide per gallon of solution and this produced a solution having a pH of 1.60.- A- panel processed in this solution, under the same. conditions, produced a coating weight of 56 mg./sq. ft. An analysis of this solution after the coating was formed showed that the solution contained 0.33% fluoride, 0.15% aluminum, and had an F/CrO ratio of 0.24. A sixth portion of the standard solution was modified to contain 89 ml. of 50% sodium hydroxide per gallon of solution and this produced a solution having a pH of 1.64. A panel processed through thissolution under the same" conditions produced a coating weight of 44 mg./sq. ft. Analysis showed this solution to contain 0.28% -F., and 0.06% aluminum.

A seventh portion of the standard solution was moditied to contain 107 ml. of 50% sodium hydroxide per gallon of solution to thus produce a solution having a pH of 1.90. A panel processed through this solution produced a coating of 38 rug/sq. ft. in '4 minutes at 120 F.

An 8th portion of the standard solution was modified to contain 125 ml. of 50% sodium hydroxide per gallon of solution and this modification formed a'solution having a pH of 2.18. A panel processed in the solution produceda coating of 22 r'ng./ sq. ft. in 4 minutes at 120 F. An analysis of the solution showed that the fluoride concentration was 0.2%.

EXAMPLE 1v An aqueous acidic solution was prepared containing 0.5% P 0.4% CrO 0.21% Al, 0.55% -F and 0.05% trivalent chromium. This solution had an F to CrO; ratio of 1.4 and a pH of about 1.46.

Clean aluminum panels processed through this solution for 4 minutes at 120 F. produced adherent green coatings having an average coating weight of 200 tug/sq. ft., and other panels treated in the same solution for 4 minutes at 135 F. were coated with anadherent green coating having a coating weight of about 280 rug/sq. ft.

. EXAMPLE V An aqueous acidic solution was prepared containing 1. P04. .25% C-rO; and 0.14% F. This solution a 'pli of 1.42.

The coating:

12 Aluminum panels of the type described above in. Ego ample I were processed through this bath at. F. for 4 minutes and additional panels. were processed until. a

total surface area of 40 square feet was treated. During the processing the coatings were adherent green coatings having weights between and mg./sq. ft., and at the end of the processing period the solution contained 0.19% aluminum, 0.06% trivalent chromium, 0.52%

fluoride, 0.25% C103, 1.5% P0 and had a pH of. 1.50 and an F to CrO ratio of 2.0.

EXAMPLE vr The use of this solution modified to contain less C103 than 3.3% produced coatings which were increasingly dusty as the proportion of C'rO was reduced. While these coatings are inferior to lighter weight more adherent coatings as a base for paint they were found to be advantageous as an aid in deformation operations, of moderate severity when preliminarily treated with an aqueous soap boraxsolution of conventional type.

EXAMPLE vn An aqueous acidic solution was prepared containing 14.5% P0 0.22% F and 0.4% CrO This solution had an F to CrO ratio of 0.55 and a pH of 1.2.

Aluminum panels processed in this solution for 4 minutes at 120 F. were coated with an adherent coating having an average coating weight of 30 mg./sq. ft.

EXAMPLE VIII Operating solutions similar to Example I, maintained at a temperature of 135 F. were used to coat a plurality of types of aluminum panels, which had been preliminarily conventionally cleaned in a dilute alkaline bath, by immersing these panels in a solution for one minute, withdrawing, water rinsing and drying. All of the panels were coated with adherent green to green-gray coatings having weights in the range of 200250 mgJsq. ft. The types of aluminum panels treated included aluminum alloys designated by the aluminum association designations numbers: 1100, 2014, 2017, 2024, 3004, 3005, 5052, 5056, 6061, 7072 and 7075.

What is claimed is:

I 1. An aqueous acidic solution for coating the surface of a metal from the class consisting of aluminum and alloys thereof in which aluminum is the principal ingredient, which consists essentially of 5-150 grams/liter phosphate ion, 2.5-6 2 grams/liter CrO 1-55 grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 123 grams/liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference the pH of the solution being between about 0.8 and 1.5, as measured by the lowest value indicated by "glasselectrode pH meter within the first 10 minutes of immersion of the electrode in the solution.

seemsand 70 grams/liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference points as follows:

Aluminum, Fluoride,

g./l. g./l.

the pH of the solution being between about 0.8 and 1:5, as measured by the lowest value indicated by glasselectrode pH meter within the first 10 minutes of immersion of the electrode in the solution.

3. An aqueous acidic solution for coating the surface of a metal from the class consisting of aluminum and alloys thereof in which aluminum is the principal ingredient, which consists essentially of -80 grams/liter phosphate ion, 5-30 grams/liter CrO 1 grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 70 grams/liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference points as follows:

-Aluminum, Fluoride, -l al the pH of the solution being between about 1.0 and 1.3, as measured by the lowest value indicated by glass-electrode pH meter within the first 10 minutes of immersion of the electrode in the solution.

' 4. A process for coating the surface of a metal of the class consisting of aluminum and alloys thereof in which aluminum is the principal ingredient which comprises the step of contacting the surface of said metal with an aqueous acidic solution consisting essential of 5-150 grams/liter phosphate ion, 2.5-62 grams/liter CrO l-55 grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 123 grams/liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference points as follows: s 7

Aluminum, Fluoride,

g./l. g./l.

the pH of the solution being between about 0.8 and 1.5, as measured by the lowest value indicated by glass-electrode pH meter within the first 10 minutes of immersion of the electrode in the solution, and maintaining said solution in contact with said metal surface until an adherent coating is formed thereon.

5. A process for coating the surface of a metal of a class consisting of aluminum and alloys thereof in which aluminum is the principal ingredient which comprises the step of contacting the surface of said metal with" an aqueous acidic solution consisting essentially of 20-80 grams/liter phosphate ion, S-3O grams/liter C50,; 1-30 grams/ liter aluminum ion, the hydrogen ion and between about 2.5 and grams/liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference points as follows:

the pH of the solution being between about 0.8 and 1.5, as measured by the lowest value indicated by glasselectrode pH meter within the first 10 minutes of immersion of the electrode in the solution, and maintaining said solution in contact with said metal surface until an adherent coating is formed thereon.

6. A process for coating the surface of a metal of a class consisting of aluminum and alloys thereof in which 1 aluminum is the principal ingredient which comprises the. step of contacting the surface of said metal with. an-

aqueous acidic solution consisting essentially of 20-80 grams/liter phosphate ion, 5-30 grams/liter CrO ,'.1-30 grams/ liter aluminum ion, the hydrogen ion and between about 2.5 and 70 grams/liter of fluoride ion, 'saidfluoride ion varying with the aluminumconcentration with reference points as follows: 5

Aluminum, Fluoride,

g./l. g./l.

aluminum and alloys thereof in which aluminumis the principal ingredient whichcomprises the step of contacting the surface of said metal with anaqueous acidic solution consisting essentially of 5-150 grams/ liter phosphate ion, 2.5-62 grams/liter CrO l-55 grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 123; grams/ liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference ;points as follows:

Aluminum, Fluoride, g./l. g./l.

the pH of the solution-being between about 0.8 and 1.5, as measured by the lowest value indicated by glass? electrode pH meter within the first 10 minutes of immer-j sion of theelectrode in the solution, and maintaining'said solution in contact with said metal surface until. a'n a d herent coating is formed thereon and replenishing ithje coating-producing ingredients from time to timewith phosphoric acid. and chromic acid. so as to maintain. these coating-producing ingredients. in substantially the original concentrations and hydrofluoric acid within the range above set forth.

' 81A method for forming a chromium-phosphate coating on the surface of a metal from the group consisting of aluminum and alloys thereof in' which aluminum is the principal ingredient which comprises the step of contacting the surface of said metal with an aqueous acidic solution consisting essentially of -15 0 grams/ liter phosphate ion, 2.5-62 grams/liter C 1-55- grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 123 grams/ liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference points as follows:

Aluminum, 4 Fluoride, sJ s the pH of the solution beingbetween about 0.8 and 11.5, as: measured. by the lowest value indicated by glass-electrode pH meter within the first 1-0 minutes of immersion of the electrode in the solution, maintaining said solution in con.- tact with said metal surface until an adherent coating is formed thereon and a steady state aluminum concentration is established, replenishing the coating-producing in.- gredients' from time to time with phosphoricacid, chromic acid and hydrofluoric acid so as to maintain the said phosphate ion, dichromate ion and fluoride ion within the range of concentration set forth therefor, and maintaining said process in steady-state operating condition by regulating the proportion of solution withdrawn therefrom as work is processed therethrough so as to maintain the aluminum concentration in said operating solution at substantiaily its said steady state concentration.

9. A method in accordance with claim 8 wherein said steady-state operation is maintained by controlling the quantity of coating solution dragged out on the work and discarding an additional quantity of said solution to there by maintain the aluminum at substantimly its steady-state concentration.

10. A composition for use in preparing an aqueous acidic solution for coating aluminum and alloys thereof which. contains as its essential coating producing ingredients, in parts by weight, 70-280 C1 0 345-1370 H P0 (75%), 65-130 A1 0 (30.5%Al) and 480-1715 water.

11. An aqueous acidic solution for coating the surface of a metal from the class consisting of aluminum and alloys thereof in which aluminum is the principal ingrediout, which consists essentially of 5-150 grams/liter phosphate ion, 2.5-62 grams/liter CrO l-55 grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 123 grams/ liter of fluoride ion, said fluoride ion varying with the altuninum concentration with reference points as follows:

"the pH of the solution being between about 0.8 and 1.15, as measured by thelowest value indicated by glass electrode pH meter Within the first 10' minutes of immersion of the electrode in the solution, and the ratio of F/CrO is above Fluoride.

g.ll.

Aluminum.

the pH of the solution being between about 0.8 and 1.5, as measured by the lowest value indicated by glasselectrode pH meter within the first 10 minutes of immeralloys thereof in which aluminum is the principal ingre dicnt, which consists essentially of 20-80 grams! liter phosphate ion, 5-20 grams/liter CrO 1-30 grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 70 grams/ liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference points as follows:

Aluminum, Fluoride,

al al the pH of the solution being between about 1.0 and 1.3, as measured by the lowest value indicated by glass-electrode pH meter within the first 10 minutes of immersion of the electrode in the solution, and the ratio of F/ C1-C is in excess of 0.5.

14. A process for coating the surface of a metal of the class consisting of aluminum and alloys thereof. in

which aluminum is the principal ingredient which com.-'

prises the step of contacting the surface of said metal with an aqueous acidic solution consisting essentially of 5-150 grams/liter phosphate ion, 2.5-62 grams/liter CrO l-55 grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 123 grams/liter of fluoride ion, said fluoride ion varying with the aluminum concentration with reference points as follows:

Aluminum, Fluoride,

all. al

the pH of the soltuion being between about 0.8 and 1.5, as measured by the lowest value indicated by glass-electrodepH meter within the first 10 minutes of immersion of the electrode in the solution, the ratio of F/CrO is above 0.405, and maintaining said solution in contact with said metal surface until an adherent coating is f rmed thereon.

' 15. A process for coating the surface of a metal of a class consisting of aluminum and alloys thereof in Aluminum, Fluoride, -l s-l the pH of the solution being between about 0.8 and 1.5, as measured by the lowest value indicated by glasselectrode pH meter within the first minutes of immersion of the electrode in the solution, the ratio of F/CrO is above 0.405, and maintaining said solution in contact with said meter surface until an adherent coating is formed thereon.

16. A process for coating the surface of a metal of a' class consisting of aluminum and alloys thereof in which aluminum is the principal ingredient which comprises the step of contacting the surface of said metal with an aqueous acidic solution consisting essentially of 20-80 grams/liter phosphate ion, 5-30 grams/liter Cr0 1-30 grams/liter aluminum ion, the hydrogen ion and between about 2.5 and 70 grams/liter offluoride, ion, said fluoride ion varying with the aluminum concentration with reference points as follows:

Aluminum, Fluoride,

g./l. g./1.

References Cited in the file of this patent UNITED STATES PATENTS 2,438,877 Spruance Mar. 30, 1948 2,798,830 Newhard et a1. July 9, 1957 2,909,455 Newhard et a1. Oct. 20, 1959 FOREIGN PATENTS 1,105,924 France July 13, 1955 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0. 2,928,763 March 15 1960 William S. Russell et al.

It is hereb$ certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 1, line 69 for "mananese" read manganese column 2, lines 6 and 7, for "subestantially" read substanti ally line 11, for O31,5" read 0.8-1.5 column 4, line 19, for "reasons" read reason column 5;,- .line 39 for "formation" read forming line 72, for "r'elaitvely" read relatively column 13 line 418 for "essential" read essentially column 16, line 5 for "5-30 grams-liter" read 5-30 grams/liter line 28 for "5-20" read 5-30 Signed and sealed this 13th day of Sept ember 1960.,

(SEAL) Attest:

KARL H. AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents 

1. AN AQUEOUS ACIDIC SOLUTION FOR COATING THE SURFACE OF A METAL FROM THE CLASS CONSISITING OF ALUMINUM AND ALLOYS THEREOF IN WHICH ALUMINUM IS THE PRINCIPAL INGREDIENT, WHICH CONSISTS ESENTIALLY OF 5-150 GRAMS/LITER PHOSPHATE ION, 2.5-62 GRAMS/LITER CRO3, 1-55 GRAMS/LITER ALUMINUM ION, THE HYDROGEN ION AND BETWEEN ABOUT 2.5 AND 123 GRAQMS/LITER OF FLOURIDE ION, SAID FLUORIDE ION VARYING WITH THE ALUMINUM CONCENTRATION WITH REFERENCE POINTS AS FOLLOWS: 